1. Field of the Invention
The present invention relates to a capacitor charging apparatus and charging method using an inverter, a switching regulator and an inductance circuit. More particularly, this invention relates to the capacitor charging apparatus and charging method which charge in two stages. This invention particularly relates to the capacitor charging apparatus and charging method which use resonant charging to achieve a highly accurate level of voltage stability, such that the charge voltage accuracy of the energy accumulating capacitor comprising the load is highly accurate to less than approximately 0.1%.
2. Description of the Related Art
In a pulse laser such as an excimer laser, the charge of a capacitor for accumulating energy, which has been charged to a high voltage of approximately several kV to several ten kV, is discharged at high speed to a laser tube via a magnetic compressor or the like, thereby exciting laser light. In the application apparatus of the pulse laser, the higher the number of laser light excitations (i.e. the higher the number of repetitions of charging and discharging the capacitor) the greater its capability as a laser apparatus. For this reason, in recent years there have been attempts to achieve a high repetition of several kHz. Consequently, the charging apparatus of the capacitor must be capable of repeating high-speed charge/discharge to completion below several hundred xcexcs. Excimer lasers require a highly accurate level of voltage stability, detecting fluctuations in the output of laser light in each cycle and controlling the output of laser light in the subsequent cycle accordingly. Therefore, the charge voltage must be controlled in each cycle, making high-speed controllability an important feature.
FIG. 19 is a circuit diagram showing one example of the constitution of a conventional resonant charging-type capacitor charging apparatus. Reference numeral 1 represents a dc (direct current) power such as a rectifier which rectifies a commercial ac (alternating current) voltage. The output of the dc power 1 is supplied to a voltage-type bridge inverter 2 (hereinafter abbreviated as xe2x80x9cinverter 2xe2x80x9d). The inverter 2 comprises four IGBTs 4A-4D (insulated gate bipolar transistors) which feedback diodes 3A, 3B, 3C and 3D are connected to in reverse parallel.
The ac side output of the inverter 2 connects via an inductance circuit 5 to a primary winding 6A of a high-voltage transformer 6 (hereinafter abbreviated as xe2x80x9ctransformer 6xe2x80x9d), becomes an ac high voltage boosted to a predetermined value at a secondary winding 6B, is converted to a dc high voltage by a high-voltage rectifier 7 (hereinafter abbreviated as xe2x80x9crectifier 7xe2x80x9d), and is supplied to a load capacitor 8. The black dots appended to the primary winding 6A and the secondary winding 6B represent the polarities of the windings. The rectifier 7 is a bridge rectifier comprising four diodes 7A, 7B, 7C, and 7D. The inductance circuit 5 also includes the inductance leaked from the transformer 6.
Reference numerals 9 and 10 represent voltage division resistances for detecting charge voltage; the resistances 9 and 10 convert a charge voltage Vc of the load capacitor 8 to a charge voltage detect signal Vd of several V (hereinafter abbreviated as xe2x80x9cdetect voltage Vdxe2x80x9d), which is input to a voltage comparator 11. Reference numeral 12 represents a reference voltage source for setting the charge voltage, and outputs a reference voltage Vr. The voltage comparator 11 compares the detected voltage Vd with the reference voltage Vr, outputting a comparison signal Vh at the H (high) level until the detected voltage Vd reaches the reference voltage Vr and outputting the comparison signal Vh at the L (low) level when the detected voltage Vd reaches the reference voltage Vr. To prevent the output comparison signal Vh from oscillating at the switchover point, the hysteresis of the voltage comparator 11 is set to approximately 0.1% of the charge voltage. Reference numeral 13 represents an inverter controller which supplies two A-phase and B-phase opposite phase signals through AND gates 14 and 15, one signal switching ON the pair of IGBTs 4A and 4D and the other signal switching ON the pair of IGBTs 4B and 4C alternately. In FIG. 19, a pair of IGBT gate signals is shown in common in order to illustrate the path of the signals, but in reality the gate signal of the IGBTs are separately insulated from each other.
The inductance circuit 5 which includes the leakage inductance from the transformer 6, the rectifier 7, and the load capacitor 8 form a half-wave series resonant circuit. The inductance circuit 5 is usually comprised of an inductor comprising the leakage inductance of the transformer 6 and an appropriate inductance, but when the inductance required for series resonance can be obtained by using only the leakage inductance of the transformer 6, the transformer 6 alone is sufficient. When the pair of IGBTs of the inverter 2 are switched ON in resonant half-cycles, the load capacitor 8 is charged by resonance to a voltage which is approximately twice the value obtained by multiplying the dc power voltage by the transform ratio n of the transformer 6. For example, when the dc power voltage Vd is 250 V, the capacity of the load capacitor 8 (Co) is 50 nF, and the boost ratio n of the transformer 6 is 20, the charge voltage Vc becomes Vc=2xc3x97nxc3x97Vdc=2xc3x9720xc3x97250=10 kV. The time during which the pair of IGBTs are ON corresponds to the charge time, that is, a resonant half-cycle. When the charge time (i.e. the resonant half-cycle) T/2=100 xcexcs, the primary conversion value Coxe2x80x2 of the load capacitor Co becomes Coxe2x80x2=202xc3x9750 nF=20 xcexcF. Since 2 xcfx80{square root over ( )}LC=T, the inductance value L of the inductance circuit 5 becomes L=(T/2xcfx80)2/Coxe2x80x2=25.3 xcexcH. In actual conventional systems, the dc power voltage Vdc changes due to fluctuation in the commercial power voltage; for example, when the commercial power voltage fluctuates between AC 180 V to 220 V, the dc power voltage Vdc changes from 240 V to 300 V. Furthermore, the resonant effect decreases to less than twice as a result of circuit loss. For these reasons, the boost ratio n of the transformer 6 is set at more than 20, e.g. 25, and the circuit constant is set so that, when the pair of IGBTs are switched ON in a resonant half-cycle while the dc power voltage Vdc is at its lowest value, the charge voltage of the load capacitor Co is greater than the set voltage 10 kV. Therefore, the IGBT of the inverter are switched OFF when the charge voltage of the load capacitor Co has reached the set voltage 10 kV, stabilizing the charge voltage within the range of power voltage fluctuation.
Subsequently, the operation will be explained by referring to FIG. 20. In FIG. 20, symbol (1) shows the current IL of the inductance circuit 5, which is equivalent to a compound current of the current of the IGBT 4A and the diode for feedback 3A, connected in reverse parallel thereto, and the current of the IGBT 4B and the diode for feedback 3B, connected in reverse parallel thereto. In FIG. 20, the currents of the diodes for feedback 3A and 3B are shown by diagonal shading. In FIG. 20, symbol (2) shows the charge voltage Vc of the load capacitor 8, and symbol (3) shows a gate signal VgA of the IGBT 4A and 4D, and a gate signal VgB of the IGBT 4B and 4C. When the load capacitor 8 is being discharged at time t0 and the detect voltage Vd is lower than the reference voltage Vr, the voltage comparator 11 outputs an H signal, and the A-phase side signal of the inverter controller 13 passes through the AND gate 14, switching ON the pair of IGBT 4A and 4D which are on the diagonal line of the inverter 2. When the IGBT 4A and 4D are switched ON, a dc power voltage is applied to the resonant circuit and a resonant current IL flows to the inductance circuit 5, boosting the charge voltage Vc of the load capacitor 8 as shown in FIG. 20. When the charge voltage Vc of the load capacitor 8 has reached the set voltage value 10 kV at time t1, the voltage comparator 11 outputs an L-level compare signal Vh and the AND gate 14 blocks the gate signal, thereby switching OFF the pair of IGBT 4A and 4D. However, electromagnetic energy accumulates in the inductance circuit 5 as a result of the current IL flowing through the circuit up to that point, and causes an inertial current (i.e. feedback current) to flow as shown by the diagonally shaded section of FIG. 20.
The feedback current charges the load capacitor 8 while flowing back to the dc power 1 along the following path: right terminal of the inductance circuit 5xe2x80x94black-dotted terminal of the primary winding 6A of the transformer 6xe2x80x94black-dotted terminal of the secondary winding 6B of the transformer 6xe2x80x94diode 7Axe2x80x94load capacitor 8xe2x80x94diode 7Dxe2x80x94non-black-dotted terminal of the secondary winding 6B of the transformer 6xe2x80x94non-black-dotted terminal of the primary winding 6A of the transformer 6xe2x80x94diode for feedback 3Axe2x80x94positive electrode to negative electrode of dc power 1xe2x80x94diode for feedback 3Dxe2x80x94left terminal of the inductance circuit 5. The load capacitor 8 is charged by this inertial current and the charge voltage Vc exceeds the set voltage of 10 kV by xcex94V as shown in FIG. 20.
After the load capacitor 8 has been discharged to an unillustrated load at time t2, the inverter controller 13 generates a B-phase signal which passes through the AND gate 15, switching ON the IGBTs 4B and 4C on the opposite diagonal line, whereby the current IL flows in the opposite direction to the inductance circuit 5 and the transformer 6. The current of the secondary winding 6B of the transformer 6 is rectified, charging the load capacitor 8. When the inverter 2 switches ON for one cycle, the load capacitor 8 is charged twice. The advantage of this bridge inverter-type resonant charging is that the switching frequency of the IGBT need only be half the charge frequency of the load capacitor 8; for example, when using an excimer laser and the like repeating a frequency of 4 kHz, a switching frequency of 2 kHz is sufficient, resulting in little switching loss.
However, this system has problems. One disadvantage of the conventional apparatuses is that even when the IGBT are OFF, the current which was flowing to the inductance circuit 5 at that point produces magnetic energy, generating an inertial current which charges the load capacitor 8 while passing through the diodes for feedback 3A to 3D, which are connected in reverse parallel to the IGBT and the like, and is fed back to the dc power 1. Consequently, the load capacitor 8 is over-charged. Even in a series resonant inverter (unillustrated) for driving the IGBT at frequencies related to the resonant frequencies of the inductance circuit 5 and capacitors for resonance, which are connected in series with the inductance circuit 5, when the charge voltage reaches the set value and the IGBT are switched OFF, an inertial current produced by the residual electromagnetic energy of the inductance circuit 5 over-charges the load capacitor 8. That is, in a capacitor charging apparatus comprising a voltage-type inverter which uses the inductance circuit on the ac side, even when the IGBT of the inverter are switched OFF, charging continues due to the inertial current and the load capacitor 8 becomes over-charged. FIG. 21 illustrates this problem, showing how the inertial current of the inductance circuit 5 over-charges the charge voltage Vc of the load capacitor 8 by xcex94V, in spite of the fact that the inverter 2 was switched OFF when the compare signal Vh of the voltage comparator 11 has dropped to the L level.
To solve such problems of the related technology, the present inventors proposed a resonant-type capacitor charging apparatus and charging method (i.e., the first to fourth embodiments of the present application described later) using an inverter and an inductance in Japanese Patent Application No. 2000-193063 (unpublished). According to this, a switching element is provided on the secondary side of the transformer, and switches ON when the load capacitor (energy accumulating capacitor) has been charged to a set voltage, whereby the inertial current produced by the inductance bypasses the load capacitor, preventing the load capacitor from being charged. This prevents the load capacitor from being over-charged.
However, the invention according to Japanese Patent Application No. 2000-193063 also has problems, since in reality the switching element and its drive circuit are not ideal; there is delay in the operating time of the voltage comparator, which compares the charge voltage of the load capacitor with the reference voltage, delay in the operation of the switching element itself, delay in the operation of the drive circuit, and the like. The total of these delays is around several hundred ns, and the charging current continues flowing to the load capacitor during this delay in operating time, resulting in over-charging, albeit a small one. In chargers for loads which require a highly accurate level of voltage stability, such as excimer lasers, the output accuracy of the charger must be sufficiently stable that the fluctuation rate of the charge voltage of the load capacitor is less than approximately 0.1%; consequently, the small amount of over-charging becomes a problem. If the amount of over-charging were constant, the controller could anticipate this amount and adjust control accordingly, but the amount of over-charging during delay is not constant for the following reasons.
(1) Firstly, there are the effects of fluctuation in the power supply voltage. Even when the operating delay is constant, fluctuation in the power supply voltage changes the boost speed of the charge voltage, changing the amount of over-charging.
(2) Secondly, there are the effects of the residual voltage of the load Capacitor and fluctuation therein. After the load capacitor has been discharged by an excimer laser load or the like, there is a feedback current from the load side which charges the load capacitor to a low voltage, which becomes the residual voltage in the next charging cycle. In particular, in the case of resonant charging, a difference between the power supply voltage and the initial voltage of the load capacitor causes changes in the resonant current. Therefore, even when the operating delay has been anticipated, the voltage accuracy is decreased by the residual voltage of the load capacitor.
(3) Thirdly, there are the effects of changes in resistance caused by the temperature of the windings of the high-voltage transformer. The temperature of high-voltage transformers and the like increases after the charger has been operating for several hours, changing the resistance of the windings. Changes in resistance in the resonant circuit affect the charge voltage boost rate, reducing the level of voltage stability even when operating delay has been anticipated.
One conceivable way of solving these difficulties is to store data relating to fluctuation in the power supply voltage, residual voltage of the load capacitor, the amount of over-charge after the inverter has switched ON, and the like, beforehand in the CPU (central processing unit) of a computer, and to control the final charge voltage based on estimates. However, in view of the demand for high speed and the large quantity of the data, control becomes complex and it is difficult to ensure stable charge voltage accuracy. According to the present invention (described later in the first to fourth embodiments), the amount of over-charging of the load capacitor after the inverter has switched OFF can be minimized, greatly increasing the level of stability, but delay in the switching element for bypass and the drive circuit thereof makes it impossible to achieve a charge voltage of sufficient accuracy.
In addition to the disadvantages mentioned above, the related technology also has problems such as the following. For example, in a charging apparatus capable of repeatedly charging and discharging in a cycle of 4 kHz, a load capacitor, which has been charged up to the final charge voltage Vc1, discharges to a load comprising a magnetic compressor or the like in compliance with a laser shot command. A laser tube converts the output from the magnetic compressor to laser light, obtaining a stepper light source. The intensity of this laser light is detected by using a sensor, the detect signal is processed by a computer, and the next final charge voltage Vc2 of the load capacitor is calculated so that each shot of the laser light output is stable. This calculation requires a certain amount of time to perform, e.g. 180 xcexcs. Assuming that the repeat frequency has a cycle of 4 kHz, then the time in one cycle is 250 xcexcs, and assuming that the calculation time is 180 xcexcs, a time of only 70 xcexcs remains in which to charge. When the charge time decreases in this way, the temporary power of the charging apparatus must be greatly increased; this is not economical.
One method for solving this problem is a two-stage charging method in which charging commences prior to calculating and setting the charge voltage, the capacitor being charged up to approximately 50% of the minimum voltage used (hereinafter termed xe2x80x9cintermediate voltagexe2x80x9d). After charging to the intermediate voltage, charging continues to the final voltage which has been calculated and set.
Resonant charging is one such method of charging a capacitor. FIG. 22 is a circuit diagram showing one example of the constitution of a resonant charger using related technology, in which a first semiconductor switch 72, a resonant inductance circuit 73, a primary winding 75 of a transformer 74, and a second semiconductor switch 76 are connected in series from dc power 71. An energy accumulating capacitor 79 is connected to a secondary winding 77 of the transformer via a rectifying diode 78 which prevents reverse flow. Feedback diodes 80 and 81 to the dc power 71 are connected from the emitters and collectors of the first and second semiconductor switches 72 and 76.
The operation is as follows. When the first and second semiconductor switches 72 and 76 switch ON simultaneously, the resonant inductance circuit 73 and the energy accumulating capacitor 79 resonate in series, charging the energy accumulating capacitor 79 equivalently toward twice the power supply voltage. The charge voltage may be controlled by controlling the voltage of the dc power 71, or by switching the first and second semiconductor switches 72 and 76 OFF when the charge voltage has reached a target value. In the latter method, when the semiconductor switches are OFF, the current of the resonant inductance 73 charges the energy accumulating capacitor 79 while passing through the feedback diodes 80 and 81 to the dc power 71.
The advantage of resonant charging is that the switching frequency can be the same as the frequency for charging the energy accumulating capacitor. For example, when using an excimer laser and the like repeating charging and discharging in a frequency of 4 kHz, a switching frequency of 4 kHz is sufficient. In a charging apparatus which uses a converter, the semiconductor switches switch at the carrier frequency of the converter, e.g. 100 kHz, and there is considerable loss, but the two-stage charging method described above is easy. On the other hand there is a problem that, when the semiconductor switches are turned OFF during charging, the resonant current is fed back to the power, and it is not possible to charge to the set voltage for a predetermined short time, even after the switches have been turned ON again. Therefore, the resonant charging system can reduce loss, but cannot achieve high-speed charging by charging in two stages and cannot accommodate a high-frequency load.
It is an object of this invention to provide a charging apparatus comprising an inverter which uses an inductance circuit on its ac side, wherein, when the charge voltage of the load capacitor reaches a set value, the inertial current of the inductance circuit is bypassed so as not to overcharge the load capacitor, increasing the accuracy and stability of the charge voltage.
It is another object of this invention to provide a charging method and apparatus which take into consideration operating delay of the switching element for bypass and the drive circuit thereof, and have sufficient capacitor charging accuracy when the load is an excimer laser and the like requiring high accuracy and stability.
It is yet another object of this invention to provide a high-speed low-loss charging method and charging apparatus by making it possible to charge an energy accumulating capacitor in two stages by using resonant charging.
A first aspect of this invention provides a capacitor charging apparatus for charging a load capacitor, comprising an inductance circuit which provides a resonant current for charging the load capacitor by resonating with the load capacitor; and a switch circuit which cuts off the supply of an inertial current, produced by energy accumulated in the inductance circuit, to the load capacitor at a predetermined timing, the switch circuit being provided on the output side of the inductance circuit.
A second aspect of this invention provides a capacitor charging method for charging a load capacitor, comprising the steps of charging the load capacitor with a resonant current, generated by resonating the load capacitor with an inductance circuit which accumulates energy; and bypassing an inertial current, produced by energy accumulated in the inductance circuit, from the load capacitor at a predetermined timing so as to prevent the inertial current from flowing to the load capacitor, using a switch circuit, provided on the output side of the inductance circuit.
A third aspect of this invention provides a capacitor charging apparatus for charging a load capacitor to a set voltage, comprising a voltage converter which switches a dc power supply voltage; an inductance circuit which provides a resonant current for charging the load capacitor by applying the switched dc voltage and resonating with the load capacitor; and a switch circuit which short-circuits the output side of the inductance circuit when the charge voltage of the load capacitor has reached the set voltage so as to prevent an inertial current, produced by electromagnetic energy accumulated in the inductance circuit, from flowing to the load capacitor, the switch circuit being provided on the output side of the voltage converter.
A fourth aspect of this invention provides a capacitor charging method for charging a load capacitor to a set voltage, comprising the steps of charging the load capacitor by using a voltage converter to switch a dc power supply voltage, and supplying a resonant current to an inductance circuit by using resonance with the load capacitor and application of the dc voltage; and when the charge voltage of the load capacitor has reached the set voltage, switching OFF the voltage converter and bypassing an inertial current, produced by electromagnetic energy accumulated in the inductance circuit, from the load capacitor so as to prevent the inertial current from flowing to the load capacitor.
A fifth aspect of this invention provides a capacitor charging method for charging a load capacitor to a set voltage, comprising the steps of charging the load capacitor by using a voltage converter to switch a dc power supply voltage, and supplying a resonant current to an inductance circuit by using resonance with the load capacitor and application of the dc voltage; switching OFF the voltage converter when the charge voltage of the load capacitor has reached a target value, which is lower than the set voltage; further charging the load capacitor with an inertial current, produced by electromagnetic energy accumulated in the inductance circuit; and when the charge voltage of the load capacitor has reached the set voltage, switching ON a switch circuit, provided on the output side of the voltage converter, bypassing the inertial current from the load capacitor and so as to prevent the inertial current from flowing the load capacitor.
According to the present invention, after the charge voltage of the load capacitor has reached a set value, the inertial current of the inductance circuit bypasses the load capacitor so that the load capacitor is not over-charged by the accumulated energy of the inductance circuit. Therefore, the load capacitor can be prevented from being overcharged, and the accuracy of its charge voltage can be increased.
A sixth aspect of this invention provides a capacitor charging method for charging a load capacitor to a set voltage, comprising the steps of: charging the load capacitor by using a voltage converter to convert an applied dc power supply voltage to a predetermined dc voltage, and supplying a resonant current to an inductance circuit by using resonance with the load capacitor and application of the dc voltage; calculating a boost in charge voltage of the load capacitor by prediction within in an operating delay td from generating a charge stop command and the actual stop of charging, and stopping charging of the load capacitor by generating the charge stop command in anticipation of the boost in charge voltage; and short-circuiting the output side of the inductance circuit by switching ON a switch circuit, provided on the output side of the voltage converter, preventing an inertial current, produced by electromagnetic energy which has accumulated in the inductance circuit, to the load capacitor.
A seventh aspect of this invention provides a capacitor charging apparatus which charges a load capacitor to a set voltage, comprising a voltage converter which switches a dc power supply voltage; an inductance circuit which supplies a resonant current for charging the load capacitor by using resonance with the load capacitor and application of the dc voltage; a switch circuit which is provided on the output side of the voltage converter and, when switched ON, short-circuits the output side of the inductance circuit so as to prevent an inertial current, produced by electromagnetic energy which has accumulated in the inductance circuit, to the load capacitor; an ON signal generator which generates an ON signal for switching ON the switch circuit in compliance with a charge stop command; and an arithmetic circuit which calculates the charge voltage boost of the load capacitor during the operating delay between the generating of the ON signal by the ON signal generator and the stopping of charging as a result of the switch circuit switching ON, and transmits the charge stop command to the ON signal generator in anticipation of the charge voltage boost.
An eighth aspect of this invention provides a capacitor charging apparatus which charges a load capacitor to a set voltage, comprising a voltage converter which switches a dc power supply voltage; an inductance circuit which supplies a resonant current for charging the load capacitor by using resonance with the load capacitor and application of the dc voltage; a switch circuit which is provided on the output side of the voltage converter and, when switched ON, short-circuits the output side of the inductance circuit so as to prevent an inertial current, produced by electromagnetic energy which has accumulated in the inductance circuit, to the load capacitor; an ON signal generator which generates an ON signal for switching ON the switch circuit in compliance with a charge stop command; and a charge voltage detector which detects the charge voltage of the load capacitor as a charge voltage detect signal Vd; and an arithmetic circuit which calculates (Vd+tdxc3x97dVd/dt) based on the charge voltage detect signal Vd and an operating delay td relating to the switch circuit, and, when (Vd+tdxc3x97dVd/dt)=a predetermined reference voltage Vr, transmits the charge stop command to the ON signal generator.
According to the present invention, consideration is given to the operating delay of the switch circuit for bypass and the drive circuit thereof. During operation delay, the switch circuit is turned ON so that the inertial current, generated by energy which has accumulated in the inductance circuit, does not charge the load capacitor, thereby achieving sufficient capacitor charge accuracy even when the load comprises an excimer laser or the like.
A ninth aspect of this invention provides a method for charging a load capacitor in stages comprising a first step of charging the load capacitor to a predetermined voltage by supplying a resonant current, generated by resonance of an inductance circuit and a resonant capacity, from a power; a second step of cutting-off the resonant current to the load capacitor, and simultaneously maintaining the energy accumulated in the inductance circuit while circulating the energy; a third step of again charging the load capacitor to a set voltage, which is set in each charge cycle and is determined in consideration of load conditions, by supplying the resonant current to the load capacitor via the inductance circuit from the power; and a fourth step of again cutting-off the resonant current to the load capacitor, and simultaneously discharging the energy accumulated in the inductance circuit.
A tenth aspect of this invention provides a capacitor charging apparatus which charges a load capacitor to a set voltage in stages, comprising an inductance circuit which supplies a resonant current for charging the load capacitor by using resonance with the load capacitor and application of a predetermined dc voltage; a voltage converter which switches a dc power supply voltage, and is provided with a feedback circuit which, during a predetermined period, circulates energy, which has accumulated in the inductance circuit, while feeding back the energy to a dc power which generates the dc power supply voltage; a switch circuit which shuts off an inertial current, produced by the energy which has accumulated in the inductance circuit, from the load capacitor; a comparator which compares a reference voltage, set to a predetermined constant reference voltage or a variable reference voltage which is calculated from load conditions after each charge cycle, with the charge voltage of the load capacitor; a controller which firstly charges the load capacitor to an intermediate voltage corresponding to the constant reference voltage based on the comparison result from the comparator, and then stops the charging of the load capacitor, circulates the current of the inductance circuit by using the feedback circuit, and subsequently sets the variable reference voltage as the reference voltage, switches OFF the switch circuit and restarts the charging of the load capacitor, charges the load capacitor to the set voltage based on the comparison result of the comparator, and then, switches ON the switch circuit and bypasses the inertial current from the load capacitor.
In the present invention, the resonant charger allows two-stage charging, comprising a stage of circulating the energy which has accumulated in the inductance circuit until the next cycle. Therefore, the load capacitor can be charged at high speed and with low loss. Furthermore, since the switch circuit such as a semiconductor switch for bypass is provided on the output side of the voltage converter, it is possible to prevent the load capacitor from being over-charged while increasing the accuracy of the charge voltage.
An eleventh aspect of this invention provides a capacitor charging method for charging a load capacitor to a target voltage, comprising the steps of using a main charger to convert an applied dc power supply voltage to a predetermined dc voltage, and charging the load capacitor by supplying a resonant current, generated by resonance with the load capacitor and application of the dc voltage, to the inductance circuit; stopping the charging of the load capacitor by using the main charger to generate a charge stop command when the charge voltage of the load capacitor has reached a predetermined voltage near the target voltage, and preventing an inertial current, produced by energy which has accumulated in the inductance circuit, from flowing to the load capacitor by short-circuiting the output side of the inductance circuit by using a bypass switch circuit; and circuit thereafter, using an auxiliary charger, which is connected in parallel to the main charger, to charge to 100% of the target voltage and auxiliarily charge the discharge of the load capacitor.
A twelfth aspect of this invention provides a capacitor charging apparatus which charges a load capacitor to a target voltage, comprising a main charger comprising a voltage converter which converts a dc power supply voltage to a predetermined dc voltage, an inductance circuit which supplies a resonant current for charging the load capacitor by using resonance with the load capacitor and application of the predetermined dc voltage, and a bypass switch circuit which short-circuits the output side of the inductance circuit by using a charge stop command, and prevents an inertial current, produced by electromagnetic energy of the inductance circuit, from flowing to the load capacitor, the bypass switch circuit being provided on the output side of the voltage converter; and an auxiliary charger which is connected in parallel with the main charger; the main charger charging the load capacitor, and generating a charge stop command when the charge voltage of the load capacitor has reached a predetermined voltage near the target voltage, shutting off the load capacitor from the electromagnetic energy of the inductance circuit by using the bypass switch circuit; and the auxiliary charger charging to 100% of the target voltage and auxiliarily charging the discharge of the load capacitor.
In conventional methods using a main charger and an auxiliary charger, the main charger charges the load capacitor to approximately 95% of the target voltage, and the remaining portion is charged by the auxiliary charger to 100%. The auxiliary charger requires a capacity having a large maximum output in order to charge the remaining portion to 100% within a predetermined time, and the output voltage of the auxiliary charger itself must be highly accurate. However, due to the effect of inertial current, which is generated by electromagnetic energy of the inductance circuit and flows to the energy accumulating capacitor, it has not been possible to achieve highly accurate charging of the energy accumulating capacitor at less than approximately 0.1%.
In contrast, according to the present invention, a high-frequency converter-type auxiliary charger is connected in parallel to the main charger, and the load capacitor is charged by the output from the main charger, which uses electromagnetic energy of the inductance circuit. When the charge voltage of the load capacitor has reached, for example, more than 99% of the target voltage, the electromagnetic energy of the inductance circuit is shut off from the load capacitor. Thereafter, the auxiliary charger having, for example, a power capacity of 2% to 9%, or more preferably, 4% to 5% of the power capacity of the main charger, charges with high accuracy to 100% of the target voltage value. In addition, the auxiliary charger performs auxiliary charging to counter discharge of the energy accumulating capacitor caused by current leakage, detect current, and the like, thereby achieving highly accurate charging of below approximately 0.1%. Therefore, the charge voltage can be controlled easily and reliably for a long period of time, and the stability of the charge voltage of an energy accumulating capacitor which requires highly accurate voltage stability, such as an excimer laser power, can be improved to less than approximately 0.1%.